1. Field of the Invention
The present invention relates to arc discharge devices and particularly to a system for producing a high voltage initiation pulse to start the discharge in such devices.
2. Description of the Related Art
Arcjets operate by heating a gas with an electric arc and expanding the heated gas through a nozzle to provide thrust. A high voltage, on the order of 5,000-6,000 volts, is required to ignite the arcjet thruster and form an electric arc between a cathode and an anode. Once the electric arc is established, the voltage necessary to sustain that arc is much less, on the order of 100 volts. An arcjet thruster thus requires two types of power, a high voltage pulse to ignite the electric arc and a relatively lower constant voltage to maintain the arc.
Several electric starters for arcjets are conventionally used. In one, as disclosed in U.S. Pat. No. 4,766,724 to Gruber, a flyback inductor is in series with the power supply and arcjet thruster. This type of starter must be capable of supporting full load power after startup has occurred. The inductor must be capable of maintaining a fixed minimum inductance during start up and a fixed minimum current during operation. Satisfying both of these requirements with a single inductor will require a large, heavy device. This weight penalty is not desirable in aerospace and outer space applications.
In a second starter, a shunt incorporates a high voltage diode blocking device in series with the power control unit output. The diode must be capable of sustaining full output current from the power control unit after startup. Again, the suitable diode would be heavy and have high internal resistance degrading power control unit efficiency.
In a third alternative, the ignition voltage is provided from a stand alone starter circuit which is removed once the electric arc is established. While such a system is effective for ground based operations such as arc welders, the weight of the separate starter makes this approach impractical for aerospace or outer space applications.
In the flyback starter approach, an output filter inductor functions as both a filter inductor and as a starter. In the starter role, a pulse width modulated converter charges the inductor up to a required energy for breakdown with maximum output capacitance and maximum possible breakdown voltage. The required energy level determines a minimum amount of energy to be stored and therefore establishes a minimum size of the inductor.
In the filter inductor role at low switching frequencies, the inductor must be large so that start constraints do not drive the design. Increasing the switching frequency permits significantly reducing the size of the output filter inductor and thus reduces inductor weight. However, starting energy level requirements in a flyback starter prevent reduction below that necessary for startup. The starting energy level requirement is related to cable capacitance and does not decrease significantly when switching frequency is increased.
Hence, while increasing the power converter operation frequency allows the output filter inductor to be reduced in size, this reduction is limited in the conventional flyback approach.
The combined presence of both high voltage and high power stresses in the same piece of magnetics further affects the size of the flyback start inductor. A space environment, for example, makes accommodation of these two stresses difficult.
Accordingly, an improved starter is needed that minimizes the size and weight of the output filter inductor and other magnetics in arcjet discharge systems while not being subject to the limitations of current approaches.
Additionally, an arcjet discharge system is needed that will reduce starting damage to the discharge unit.
Accordingly, this invention is directed to an arc discharge system that substantially obviates one or more of the problems due to limitations and disadvantages of the prior art.
There exists a need for an electric circuit to ignite an arcjet thruster and maintain the electric arc subsequent to ignition which does not have the problems of the prior art.